Conventionally, there has been a technique for using a voltage-variable power supply as the power supply of an amplifier apparatus to make it follow the level of an input audio signal to the amplifier apparatus in order to increase or decrease the value of power supply voltage to be supplied to a power amplifier stage, thereby reducing noise superimposed on an output signal and improving the power efficiency at the power amplifier stage. In this case, the power supply voltage to be supplied to the power amplifier stage is made to follow the level of the input audio signal. Therefore, when the level of the input audio signal is low, since the power supply voltage at the power amplifier stage can be reduced to a voltage value with an amplitude that does not distort the amplified signal, noise superimposed on the output signal of the amplifier apparatus can be reduced and power efficiency at the power amplifier stage can be improved.
However, when the power supply voltage to be supplied to the power amplifier stage is lowered, the amplification gain of the amplifier apparatus due to voltage amplification at the power amplifier stage also drops. To prevent this, there is known a technique in which a constant voltage control circuit controls the voltage of a power amplification unit in proportion to the amplitude level of an input analog signal, and an attenuator provided in a feedback circuit cooperates with the constant voltage control circuit to adjust a feedback amount to be kept constant according to the voltage level of a constant voltage (for example, see Patent document 1 cited below). There is also known another conventional technique in which the gain of gain adjustment means provided in a feedback loop is adjusted according to an increase or decrease in voltage value to be supplied to a switching amplifier in a signal reproducing apparatus (for example, see Patent document 2 cited below).